A Low Pass Gate (LPG) is combination of Low Pass filter (LPF) and Voltage controlled amplifier based on one of Don Buchla's designs. Often LPGs can be switched between low pass and VCA modes, i.e. three modes of operation when the LPG mode is counted, although the module may act only as a filter/LPG or some other more limited combination.
The real wonder of an LPG only become fully apparent when in actual LPG mode – a combined LP+VCA mode in which the sound becomes brighter as the loudness increases. This 'colouring' of the VCA action by the LPF is generated using a component called a vactrol – an electronic device which transfers electrical signals using light waves to 'bridge an electrical gap' between its input and output, which has a slow enough response as far as electronics are concerned that it has an audible effect on the timbre of the output sound.
Some LPGs employ a resonant LPF which can impart even more colour to the sound, although the classic Buchla LPG sounds such as the "Buchla Bongo" rely on frequency modulation, Waveshaping/Folding or distortion to create a richer spectrum of sound to the input, allowing more scope for the filter to add colour as it opens, allowing harmonics/other extra audio frequencies through.
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